Huy Sokchea, Khieu Borin and T R Preston*

Abstract

The objective of this study was to measure the
effect of biochar produced from rice husk by different types of combustion (drowndraft
gasifier and paddy rice drying machine) and the interaction between two kinds of
fertilizer (biodigester effluent and urea) on soil fertility and paddy rice
grain yield. The experiment was done at the ecological farm of the Center for
Livestock and Agriculture Development (CelAgrid), located in Phnom Penh city,
Cambodia. The experiment was designed as a 2*2*2 factorial in a completely
randomized block design (CRBD) with 4 replicates and in 32 plots each of 20 m2.
The first factor was type of biochar (from a downdraft gasifier or a rice
dryer); the second factor was the level of biochar (0 and 3 kg/m2);
the third factor was source of fertilizer N (Biodigester effluent or urea at 100
kg N/ha/crop).

The rice husk biochar increased yields of rice
grain and straw by 30 and 40%, respectively; but there were no differences
between biochar produced in a downdraft gasifier compared with that from a rice
dryer, nor between urea and biodigester effluent as N fertilizer. Biodigester
effluent increased rice grain yield more than urea in the absence of biochar but
there were no differences between the two fertilizers when biochar was applied.
Biochar increased soil pH, water holding capacity and cation exchange capacity.
These criteria were not affected by the source of N fertilizer, nor by the
source of the biochar.

Introduction

The population of Cambodia was
almost 15.1 million in 2010, and will increase to 23.8 million in 2050 but with
40 percent still being under the poverty line (PRB 2010). Poverty, population
growth and environmental degradation (air, soil and water pollution) are
increasingly being considered as major subjects for research and development.
Agriculture is very important in Cambodia with around 37.1% of GDP generated
from agricultural productivities (FAO 2003). Soil is one of the most important
factors in determining crop yields. For agriculture to be sustainable there is
an immediate need to combat the problem of soil erosion and to increase food
production. According to MAFF (1996), soil fertility depends on the
agro-ecosystem. There are four important rice agro-ecosystems in Cambodia: rain
fed lowland rice, rain fed upland rice, deep-water or floating rice, and
dry-season (flood recession) rice. There are 2.3 million ha in lowland rice in
Cambodia but most of the soils are sandy and poor in nutrients. Erosion occurs
not only in the upland areas but also in the lowland areas. In practice, water
run-off occurs on all land, and the top soil is lost when no protective and
conservation measures are in place. The most common rain-fed lowland soils are
sandy, acidic, extremely infertile and low in organic carbon and cation exchange
capacity.

Global climate change raises
major questions about management of fibrous residues from rice growing – straw
and rice husks. Decomposition of organic matter in flooded rice gives rise
to emissions of methane, which is about 22 times more climate forcing than CO2.
Rice-based systems are estimated to contribute from 9 to 19% of global methane
emissions. An opportunity to address these issues in a completely new way arises
from research on anthropogenic soils in Brazil, called terra preta. These soils
are characterized by high content of black carbon (carbonized organic matter or
biochar) most probably due to the application of charcoal, according to Sombroek
(1966).

Agricultural fires were found
to account for 8-11% of the annual global fire activity. Burning crop residue
before and/or after harvest is a common farming practice. About 30% more GHG
emissions can be reduced when the biochar is applied to soil. The biochar option
can address issues emerging from soil organic carbon depletion and carbon
sequestered in soil actually removes CO2 from the atmosphere. Biochar
formation decelerates the carbon cycle with important implications for carbon
management. Terra Preta may be the best proof that soil organic carbon (SOC)
enrichment is possible if done with a form of carbon such as biochar. Terra
Preta soils show not only a doubling in the organic carbon content but also a
higher cation exchange capacity (CEC) (Jonah et al. 2010).

Materials and methods

The experiment was arranged as a 2*2*2 factorial in a
completely randomized block design (CRBD) with plot size 20 m2 (4*5m)
and 4 replicates. The first factor was type of biochar (from gasifier or paddy
rice dryer machine); the second factor was fertilizer (biodigester
effluent or urea); the third factor was level of biochar (0 and 3 kg/m2).
There were 32 plots in total with the overall area of 640 m2 (Table
1).

Table 1:
Layout of the experiment

1

2

3

4

5

6

7

8

GUB3

SUB3

GEB3

SUB0

SEB3

GUB0

GEB0

SEB0

9

10

11

12

13

14

15

16

SUB0

GUB0

SEB0

SEB3

GUB3

GEB3

SEB0

GEB0

17

18

19

20

21

22

23

24

GEB0

GUB3

GUB0

SEB0

GEB3

SEB3

SUB3

SUB0

25

26

27

28

29

30

31

32

SUB3

GUB0

SEB3

GEB0

GEB3

SUB3

GUB3

SUB0

Experimental materials

The rice seeds were bought from DomnukTeuk group, Phnom
Penh, Cambodia. The urea was bought from the local market while the effluent was
produced by a concrete dome biodigester, charged with pig manure. The biochar
(dryer) was collected from the rice grain dryer in CelAgrid farm; the
biochar (gasifier) was bought from the local rice milling station.

Photo 1: Biochar from rice dryer

Photo 2:
Biochar from downdraft
gasifier

Biochar

The
biochar (dryer) was obtained from a machine used to dry paddy rice (Photo 3).
The feedstock used in the furnace of the dryer was rice husk. The temperature in
the furnace was around 500 0C. The other source of biochar was a
commercial down-draft gasifier (Photo 4) producing a combustible gas
(approximately 20% hydrogen and 20% carbon monoxide) which was used as fuel in
an internal combustion gas engine driving an electrical generator to produce
electrical power for the rice mill. The temperature inside
the gasifier was around 6000C.

Photo 3:
Dryer machine utilizing rice husk as feedstock

Photo 4: Downdraft gasifier, designed for
rice husk feedstock

The biochar was sprayed on the flooded soil surface
(Photo 5), and immediately afterwards the plots were ploughed to break down the
large particles of soil and to ensure the texture was suitable to transplant the
germinated rice.

The biodigester effluent and the urea were applied in three
steps: the first time was 25 days after transplanting the rice, and then after
two successive intervals of 20 days. The total quantity was the equivalent of
100 kg N/ha. The effluent was pumped from the biodigester to PVC drums
situated in each block (Photo 7). From the drums the effluent was applied by
hand. Urea was broadcast by hand. The plots were irrigated with water from a
well. The amounts applied were sufficient to maintain the water levels in the
plots (Photo 8).

Photo 7:
Effluent flow
system

Photo 8:
Water supply from the well

Planting and harvesting of the rice

The rice seed variety name was Phka Romdoul and it was
sown in a nursery for germination. After 20 days, it was transplanted in the
experimental plots. Two plants were planted in each hole which was at 30 cm
distances. Midway through the growing season (50 days) the tillers were counted
on 16 randomly selected plants in each plot (Photo 9).

At the time of harvest (Photo 10), the rice plants from
each plot were gathered (Photo 11) and separated into grain and stems + leaves
which were weighed separately.

The rice grain and straw (stems + leaves) were analysed for
DM by the micro-wave radiation method of Undersander et al (1993). Nitrogen and
ash were determined following AOAC (1990) procedures. Organic carbon was
calculated as OM/1.724 (Walkley et al 1934). Soil
samples were analysed for texture, separating the fractions into clay, fine
silt, coarse silt, fine sand and coarse sand using the Pipette Method (Day
1965). The cation exchange capacity (CEC) was determined by titrating
with 1M Calcium Chloride at pH 7 (Rhoades1982).
The water holding capacity was determined by weighing 15 g of soil into a glass
funnel fitted with filter paper and then saturating the soil with water (Photo
12). After 24 h the soil was weighed to determine the quantity of water that had
been retained.

Photo 12:
Adding water to saturate
the soil then allowing the water to drain for 24 hours to determine
water holding capacity

For measurement of the pH, the soil samples were dried
in the microwave oven, then ground to a powder. Five grams of the ground sample
was put in a beaker and 25 ml of distilled water were added. The suspension was
stirred 3 times at 15 minute intervals, and then filtered. pH was measured on
the filtrate using a digital pH meter.

The data were analyzed by the
General Linear Model of the ANOVA program in the Minitab software (Minitab
2000). Sources of variation were: Biochar source, fertilizer source, biochar
level and interaction biochar source*fertilizer source*biochar level and error.

Results and discussion

According to Turenne (2011),
soil texture is determined by the size of the particles: very coarse sand:
2.0-1.0 mm, coarse sand: 1.0-0.5 mm, medium sand: 0.5-0.25 mm, fine sand:
0.25-0.10 mm, very fine sand: 0.10-0.05 mm, silt: 0.05-0.002 mm and clay: <
0.002 mm. There are three elements that define soil type: texture, structure,
and porosity. Soil texture is determined by the percentages of sand, clay and
silt while soil structure is the way the clay, sand and silt particles join
together with organic matter to form aggregates or clusters of particles. The
data in Table 2 indicate that the soil in the experimental area would be
classified as “loam” soil (Berry et al 2007).

Table 2:
Soil texture, using the Pipette Method

Clay

Fine silt

Silt

Fine sand

Sand

8.6

53.2

12.6

18.5

6.3

Table 3:
Chemical composition of biochar, biodigester effluent and soils (soil
samples were taken at the beginning of the experiment after application
of biochar and fertilizer)

OM, %

OC, %

DM, %

pH

N, mg/liter

P,
%

K,
%

CEC, meq/100g

Gasifier
biochar (GB)

53.9

31.2

61.9

9.8

N/A

N/A

N/A

69

Dryer biochar
(DB)

10.3

5.99

91.7

10.7

N/A

N/A

N/A

78

Effluent

N/A

N/A

N/A

5.8

400

0.12

0.10

N/A

Soil

14.6

N/A

88.9

5.5

0.15

N/A

N/A

N/A

N/A= Not analyzed

Soil pH was increased by
application of biochar (Table 4) as was tillering capacity. Agusalim (2010) also
showed that the application of rice husk biochar as a soil amendment could
increase the number of rice tillers, compared to untreated soil. The water
holding capacity of the soil was increased by application of biochar but there
were no differences between the sources of biochar nor between urea and
biodigester effluent fertilizers (Table 4). These results are similar to those
reported by Agusalim (2010) where water holding capacity was increased from
11.3% for untreated control soil to 15.5% for soil treated with rice husk
biochar. Sokchea et al (2011) and Sisomphone et al (2011) reported increases in
WHC of soil from 43 to 53% and 40 to 50%, respectively, as a result of biochar
application. The higher values in these latter reports probably reflected
differences in soil characteristics between the different experiments.
Lehmann et al (2009) suggested that biochar application may enhance the water
holding capacity of the soil, and Chan et al (2007) also showed that biochar
application in the soil improved some physical properties of soils, such as
increased soil aggregation and water holding capacity.

Table 4: Mean values for number
of tillers, and pH and water-holding capacity (WHC) of the soil
according to source of biochar, level of biochar, and source of
fertilizer (measurement of tillers was done midway through the
experiment; measuurements on soils were taken at the beginning
of the experiment after application of biochar and fertilizer)

Biochar source(BS)

Biochar level
(BL)

N source
(N)

Probability

Dryer (D)

Gasifier(G)

None

3 kg/m2

Effluent(E)

Urea

SEM

BS

BL

N

Tillers/plant

14.70

15.45

13.10

17.08

14.65

15.53

2.229

0.353

0.000

0.288

Soil pH

5.80

5.72

5.49

6.03

5.81

5.71

0.090

0.528

0.000

0.770

WHC, %

15.2

14.7

12.1

17.8

14.5

15.4

1.264

0.770

0.004

0.585

At the beginning of the experiment and after application of biochar, the cation
exchange capacity (CEC) was increased by both kinds of biochar (Table 5a; Figure
1a). Content of calcium, sodium and magnesium were not affected by biochar
addition but content of potassium was increased two-fold. However, in the
samples taken after harvest (Table 5b; Figure 1b) there appeared to be no effect
of the biochar on the CEC, while the content of the calcium, sodium and
magnesium were increased, while that of potassium had decreased. As in the
samples taken at the beginning of the experiment, availability of potassium was
increased by bochar with no effect on the other elements. We have no explanation
for the changes in cation status which appeared to have occurred in the soils
after harvest.

According
to Lehmann (2003) the availability of potassium, phosphorus and zinc are
upgraded when biochar is applied but calcium and copper less so Increase in CEC
of up to 40% over initial CEC by addition of biochar was reported by Topoliantz
(2002). James et al (2010) also showed that biochar increased the CEC of
the soil, and that this was associated with soil fertility improvement and
decreased fertilizer runoff. Many authors (Liang et al 2006; Yamato et al 2006;
Priyadarshini et al 2010 and Agusalim (2010) have reported increases of CEC in
soils through application of biochar.

Table 5a:
Exchangeable cation content and cation exchange capacity (meq/100g soil)
on soils after treatment without(B0) or with 3% biochar (B0)
(from gasifier or dryer) and fertilized with biodigester effluent
or urea at the beginning of the experiment

Incorporating biochar in the soil increased yields of grain
and straw by 30 and 40%, respectively (Table 6; Figures 2 and 3); but there were
no differences between the two sources of biochar, nor between urea and
biodigester effluent as fertilizer.

Table 6: Mean
values for yield of grain and straw (kg DM/ha) according to source
of biochar, level of biochar, and source of fertilizer

Increases in rice yield from
application of biochar were reported by Bounsuy (2010) in Cambodia. He recorded
yields of 3.76 tonnes/ha with application of 40 tonnes/ha of biochar compared
with 1.82 tonnes/ha with 20 tonnes/ha of biochar. Priyadarshini et al
(2010) described linear increases in rice yield from application of biochar.
According to Afeng et al (2010), biochar amendment at 10 and 40 tonnes/ha
increased the rice yield by 12% and 14% in unfertilized soils and by 8.8% and
12.1% in the soil with N fertilization. However, Singhal et al (2011) showed
that application of 2 tonnes rice-husk-biochar per ha increased the grain yield
from less than 4 tonnes per ha for the control treatment to more than 5 tonnes/ha
for the biochar treatment.

There were no interactions
between the effects of level of biochar and source of fertilizer on tillering
rates of the rice plants and soil pH (Table 7; Figures 4 and 5). Tillering was
increased by effluent compared with urea when no biochar was applied but there
were no differences between the two fertilizers in the presence of biochar. In
the absence of biochar, grain yield was higher with effluent but the contrary
was the case when biochar was applied. Soil pH showed the same trends as grain
yield. It was to be expected that grain yield would be higher with effluent as,
besides nitrogen, this fertilizer also contained a range of other plant
nutrients.

Table 7: Mean values for
numbers of tillers, soil pH (at beginning after application of
biochar and fertilizer) and rice yield, according to application of
biochar and source of fertilizer (Interaction effects)

Conclusions

Incorporating 3 kg/m2 of rice husk biochar in a loam
soil (p increased pH 5.5) increased yields of rice grain and straw by 30 and
40%, respectively. However, there were no differences between biochar
produced in a downdraft gasifier compared with that from a rice dryer, nor
between urea and biodigester effluent applied at 100 kg N/ha.

Biodigester effluent increased rice grain yield more
than urea in the absence of biochar but there were no differences between
the two fertilizers when biochar was applied.

Biochar increased soil pH, water holding capacity and
cation exchange capacity in the soils at the beginning of the experiment,
but had no effect in the samples taken after harvest. These criteria were
not affected by the source of N fertilizer.

Acknowledgement

The authors express their appreciation to the MEKARN
program, financed by Sida (Sweden), for the grant which made possible this
research, and to staff members and students in CelAgrid for their assistance in
the experimental work. The senior author is especially grateful to the ěRSKOV
Foundation for funds which facilitating his participation in the Cantho
University MSc course, for which this research paper in one of the requirements.